The present invention relates to the field of LED based lighting and more particularly to a fault detection mechanism for lighting based on a series LED string.
Light emitting diodes (LEDs) and in particular high intensity LED strings are rapidly coming into wide use for lighting applications. High intensity LEDs are sometimes called high power LEDs, high brightness LEDs, high current LEDs or super luminescent LEDs and are useful in a number of lighting applications including backlighting for liquid crystal display (LCD) based monitors and televisions, collectively hereinafter referred to as a monitor. In a large LCD monitor typically the high intensity LEDs are supplied in a string of serially connected high intensity LEDs, thus sharing a common current.
In order to supply a white backlight for the monitor one of two basic techniques are commonly used. In a first technique one or more strings of “white” LEDs are utilized, the white LEDs typically comprising a blue LED with a phosphor which absorbs the blue light emitted by the LED and emits a white light. In a second technique, individual strings of colored LEDs are placed in proximity so that in combination their light is seen a white light. Often, two strings of green LEDs are utilized to balance one string each of red and blue LEDs. Unfortunately, in either of the two techniques, in the event of a failure of a single LED in the string to conduct electricity, i.e. an open LED failure, the entire LED string fails to operate. An LED string is costly, and is typically only supplied today in high end LCD based monitors. Thus, disadvantageously according to the prior art, failure of a single LED in an LED string causes a partial failure of a high end LCD monitor.
In either of the two techniques, the strings of LEDs are typically located at one end or one side of the monitor, or in zones behind the monitor, the light being diffused to appear behind the LCD by a diffuser. In the case of colored LEDs additionally a mixer is required to ensure that the light of the colored LEDs are not viewed separately, but are rather mixed to give a white light. The white point of the light is an important factor to control, and much effort in design and manufacturing is centered on the need for a correct white point.
U.S. Patent Application Publication S/N US 2005/0231459 A1 published Oct. 20, 2005 to Furukawa is addressed to a constant current driving device for constant current driving of a plurality of elements connected in series with each other by a pulse width modulation constant current driving circuit includes: switching elements respectively connected in parallel with the plurality of elements connected in series with each other; a control circuit for performing control to bypass a driving current flowing through the other elements than an arbitrary element to be measured via the respective switching elements and pass a measuring driving current through only the element to be measured; and a detecting circuit for identifying an element at a faulty position by detecting the driving current flowing through the plurality of elements connected in series with each other.
Such a mechanism however requires bypassing the LEDs, with the exception of the LED being tested, which interferes with normal operation. Additionally, such a detection control unit is expensive, in that it requires an active switching element in parallel with each LED. Furthermore, in the event that strings of colored LEDs are supplied, no mechanism to compensate for lack of color balance, i.e. shift in white point, is provided and the LCD monitor will thus exhibit an improper color balance
There is thus a long felt need for a simple fault detection mechanism capable of identifying a fault in an LED string. There is further a need for supplying a means of chromatic compensation for a failed colored LED in a backlighting string of an LCD monitor.